Everyday alchemy, coffee roasting coaxes golden flavor from a bland bean. Unroasted beans boast all of coffees acids, protein, and caffeinebut none of its taste. It takes heat to spark the chemical reactions that turn carbohydrates and fats into aromatic oils, burn off moisture and carbon dioxide, and alternately break down and build up acids, unlocking the characteristic coffee flavor.
A note on flavor: Describing the tastes of different roasts is as subjective as putting a wine into words. In both cases theres no substitute for your own personal taste, so sample away! (Many descriptions below are based on Kevin Knox and Julie Sheldon Huffakers Coffee Basics: A Quick and Easy Guide.)
Aliases: Cinnamon roast, half city, New England
Roaster Watch: After about seven minutes the beans pop and double in size, and light roasting is achieved. American mass-market roasters typically stop here.
Flavor: Light-bodied and somewhat sour, grassy, and snappy
Aliases: Full city, American, regular, breakfast, brown
Roaster Watch: At nine to eleven minutes the beans reach this roast, which U.S. specialty sellers tend to prefer.
Flavor: A bit sweeter than light roast; full body balanced by acid snap, aroma, and complexity
Aliases: High, Viennese, French, Continental
Roaster Watch: After 12 to 13 minutes the beans begin hissing and popping again, and oils rise to the surface. Roasters from the U.S. Northwest generally remove the beans at this point.
Surface: Slightly shiny
Flavor: Somewhat spicy; complexity is traded for rich chocolaty body, aroma is exchanged for sweetness
Aliases: Italian, espresso
Roaster Watch: After 14 minutes or so the beans grow quiet and begin to smoke. Having carmelized, the bean sugars begin to carbonize.
Surface: Very oily
Flavor: Smokey; tastes primarily of roasting, not of the inherent flavor of the bean